A mechanistic study on the S_Hi reaction at tin atoms in a radical cascade reaction

Abstract

A kinetic study on the intramolecular direct radical substitution reaction on tin atoms was undertaken.

Comparison of homofugality among alkyl groups attached to tin atom

“…Stereochemistry of stannolane 2b and 2c was determined by NMR analyses. Firstly, we thought that all of the configuration of compound 2 except for that on the tin atom must be 3S,3aS as illustrated in Scheme because the cascade reaction always progresses in a highly stereoselective manner [5], [6] [9]. In the previous results, we found that an alkyl group on tin atom shows downfield shift if the group located in the cis ‐position to the ester group at the bridgehead carbon.…”

“…In the previous results, we found that an alkyl group on tin atom shows downfield shift if the group located in the cis ‐position to the ester group at the bridgehead carbon. This is probably because of anisotropic effect from the ester group [6] . This tendency was useful to determine the stereochemistry of 2b and 2c .…”

“…The butyl group was substituted by vinyl radical intramolecularly to form five‐membered ring containing tin atom, stannolane. The reaction rate of the intramolecular radical substitution is quite fast, and its estimated rate constant reaches 10 8 M −1 order [6]. When the same precursor 1 was treated with dibutylphenyltin hydride instead of tributyltin, small amount of piperidine was formed along with the formation of stannolane derivatives (Scheme , b).…”

“…Configuration of 3a,5‐ trans ‐ 2d , reported as 3c‐A in ref. [6], was confirmed by X‐ray crystallographic analysis (CCDC 1050624).…”

“…For compound cis ‐ 2d , reported as 3c‐B in ref. [6], 1 H NMR signals for the terminal methyl group in the butyl group on the tin atom, appearing at 0.87 ppm, shows downfield shift when the butyl group is located in cis position to the ester group at the bridgehead carbon. The corresponding trans ‐isomer showed the same proton at 0.76 ppm.…”